You don´t need to. Unless you want to simply show it off but there´s no doubt it will stick to a roof over a given speed.

Windtunnels have scales on each tire. Crank up the wind tunnel and you get data on just how much downforce the car is producing roughly, give or take for real world always being different in the finer details.

The cars weigh around 660kg and produce well over a 1000kg of downforce if i´m not mistaken.

So, instead of using engineer's time for things electric car developments and making motor-sport more eco-friendly let's put a car upside down for a few seconds where a slight driver error could cause death. Sounds like a plan...